1. Field of the Invention
The present invention relates to a semiconductor device for controlling electricity and, more particularly, to the semiconductor device of kind including an insulating substrate having a back-side metal pattern on its back face, which pattern is bonded to a metal base plate by a binder, and a pair of circuit patterns on its front face.
2. Description of the Prior Art
IPM (Intelligent Power Module) has been known as a semiconductor device for controlling electricity and is used in, for example, a drive control of an electric car. However, a need has been recognized for the electricity control device that has a longer life and a higher reliability in severe conditions of heat or vibration. FIGS. 4 through 7 show an example of such a conventional semiconductor device for controlling electricity.
In FIGS. 4 through 6, reference numeral 1 denotes a generally rectangular metal base plate made of, for instance, a copper-molybdenum alloy. This metal base plate 1 has boltholes each defined in a corner region thereof for attachment of the metal base plate 1 to a radiating fin assembly (not shown in the Figures). Reference numeral 2 denotes insulating substrates fixed on the metal base plate 1 by means of soldering. Each of those insulating substrates 2 consists of an insulator plate 3 made of aluminum nitride, a pair of copper circuit patterns 4 formed on a front face of the insulator plate 3 and a back-side pattern 5 formed on a back face of the plate 3. Each insulating substrate 2 is fixed on the metal base plate 1 with the back-side pattern 5 soldered thereto. Reference numeral 6 denotes insulated gate bipolar transistors (IGBTs) as switching semiconductor devices for controlling electricity, and reference numeral 7 denotes free-wheel diodes (hereinafter, referred as FWDs) each paired with the adjacent IGBT 6. A pair of IGBT 6 and FWD 7 are soldered on each of the circuit patterns 4. Reference numeral 9 denotes a solder layer interposed between the metal base plate 1 and the back-side pattern 5 of the insulating substrate 2, and reference numeral 9a denotes a solder layer interposed between each circuit pattern 4 and the associated pair of IGBT 6 and FWD 7. Reference numeral 10 denotes a pair of thermistors mounted on a center area of each insulated substrate 2 between the circuit patterns 4 of the associated pair for detecting the temperature of such insulating substrate 2.
Reference numeral 11 denotes a resinous cover having boltholes 11a defined therein in alignment with the respective boltholes in the metal base plate 1 for attachment thereof together with the metal base plate 1 to the radiating fin assembly (not shown). The metal base plate 1 and the cover 11 are assembled together to define a generally rectangular box-like case with the base plate 1 serving as a bottom wall, and the insulating substrates 2, IGBTs 6 and FWDs 7 on the metal base plate 1 are thus encased within the case so defined. Reference numeral 12 denotes main circuit terminals inserted into the cover 11. Each of those main circuit terminals has outer and inner ends with the outer end positioned outside the case and with the inner end connected to the associated IGBT 6 and FWD 7 by means of aluminum wires 13. Reference numeral 14 denotes electrodes for a controlling circuit, and reference numeral 15 denotes a cover plate for the above-described case. It is to be noted that respective connections between the electrodes 14 and the controlling circuit board are not shown.
It is to be noted that the terminals employed in the illustrated electricity control device and generally identified by 12 as described above are many, but have different functional attributes. Accordingly, the terminals 12 have respective suffixes annexed (P), (N), (U), (V) and (W) thereto to show that the terminals 12(P) and 12(N) serve as positive and negative input terminals, respectively, and the terminals 12(U), 12(V) and 12(W) serve as respective three-phase output terminals, i.e., U-phase, V-phase and W-phase output terminals, respectively. In addition, symbols (U), (V) and (W) affixed to the reference numeral 2 to denote each of the insulating substrates 2 in general are intended to show that the insulated substrates 2(U), 2(V) and 2(W) are those associated respectively with three phase switching circuits each comprised of the corresponding IGBT 6 and the FWD 7 connected parallel, but in reverse relation to such IGBT 6. Symbols (H) and (L) affixed to the reference numeral 4 used to generally identify each circuit pattern denote respective two switching devices in each switching circuit in higher and lower voltage side, respectively.
FIG. 7 shows a circuit diagram of a main inverter circuit included in the semiconductor device for controlling electricity shown in FIGS. 4 through 6. Even in the circuit diagram of FIG. 7, similar symbols P, N, U, V and W are employed whicyh correspond respectively to 12(P), 12(N), 12(U), 12(V) and 12(W). Similarly, symbols 2(U), 2(V), 2(W), 4(H) and 4(L) used in FIG. 7 correspond to 2(U), 2(V), 2(W), 4(H) and 4(L) used in FIGS. 4 through 6, respectively.
Arrangement of IGBTs 6 and FWDs 7 on the insulating substrates 2, mounted on the metal base plate 1, will now be described. In FIGS. 4 through 6, the three insulating substrates 2 are disposed on the metal base plate 1 in line with each other and spaced a predetermined distance from each other. Each substrate 2 is soldered on the metal base plate 1 via the back-side pattern on the back face of the substrate. The solder layer 9 is formed between the metal base plate 1 and the back-side pattern on each substrate 2. Each pair of circuit patterns 4 on the front face of the insulating substrate 2 are placed above the back-side pattern 5 via the associated insulator plate 3.
Each circuit pattern 4 is of a generally L-shaped configuration extending in part along one of four sides of the corresponding insulator plate 3 and in part along another one of the four sides thereof which is continued from and lies perpendicular to such one of the four sides of the corresponding insulator plate 3. Two L-shaped circuit patterns 4 on the respective insulator plate 3 are disposed centrosymmetrically with respect to each other. In each circuit pattern 4, IGBT 6 is positioned near the corner, FWD 7 is next to IGBT 6 along one side, and electrode-pattern region 4a is along another side of the insulator plate 3. That is, IGBT 6 and the electrode-pattern regions 4a are placed alternately on the metal base plate 1 so that the inner ends of the main circuit terminals 12 extending into the case are connected the shortest distance with IGBT 6 and the electrode-pattern regions 4a. A pair of thermistors 10 for detecting the temperature of the insulating substrate 2 are disposed between the two circuit patterns 4 at a location generally in alignment with the center of the insulating substrate 2. Hereinafter, the function of the device will be described. During a current flowing in the main circuit, IGBT 6 repeats switching motion, and IGBT 6 and FWD 7 generate a heat which is then transferred to the metal base plate 1 through the solder layer 9a, the circuit pattern 4, the insulator 3, the back-side pattern 5 and the solder layer 9. The heat transmitted to the metal base plate 1 is diffused to the radiating fin assembly (not shown) attached to the metal base plate 1.
During the heat transfer, the solder layer 9 that is used to connect the back-side pattern 5 with the metal base plate 1 suffers from a complicated heat stress. The heat stress is caused by a variety of reasons; a difference in thermal-expansion coefficient between the metal base plate 1 and the insulator plate 3, which is combined with the back-side pattern 5; a temperature gradient between the metal base plate 1 and the back-side pattern 5; and a spatial variation of the temperature gradient caused by a temperature distribution in the back-side electrode 5, in which a part near the highly-heat-generating IGBT 6 has higher temperature than other parts.
The conventional semiconductor device of the structure discussed above has numerous problems as follows. Firstly, small cracks may occur in the solder layer 9 by heat cycles that inevitably arise when the device is set in an engine compartment or that is generated by a start-and-stop operation of IGBT 6. The small cracks generally start from corners of the back-side pattern 5. At the corners of the pattern 5, a distance from a center of the back-side pattern 5 and, thus, an amount of an expansion or shrinkage by heat is maximum. Also, strains are apt to concentrate at the corners. The generated cracks run toward the center of the back-side pattern 5. When the cracks in the solder layer 9 reach under a part of the back-side pattern 5 under IGBT 6, a heat-radiating ability of this part is reduced. This may result in heat breaking of IGBT 6.
Secondly, IGBT 6 placed near the corner of the back-side pattern 5 may be extraordinarily heated under influence of the small cracks that have developed from the corners of the back-side pattern. Although the thermistors 10 are mounted in the center area of the insulating substrate 2 for detecting the temperature of the substrate 2, the thermistors 10 are located too far from the heating-up point to detect the heat accurately. This renders it difficult to grasp the lifetime of the device.
It is accordingly an object of the present invention to provide a semiconductor device for controlling electricity that is less susceptible to small cracks in a solder layer between a metal base plate and a back-side pattern of an insulating substrate, and capable of extending the lifetime and that can be assembled compact.
It is another object of the present invention to provide a semiconductor device for controlling electricity that has a high detecting ability for an extraordinary heating of a switching semiconductor device mounted on a insulating substrate, and is capable of predicting the lifetime of the device highly accurately.
In accordance with a first aspect of the present invention, a semiconductor device for controlling electricity includes:
(a) a metal base plate; and
(b) at least one insulating substrate including
(1) an insulator plate,
(2) a back-side pattern on a back face of the insulator plate, the back-side pattern being bonded to the metal base plate and
(3) two circuit patterns located on a front face of the insulator plate and above the back-side pattern, each of the circuit patterns including a semiconductor switching element for controlling electricity, a free-wheel diode paired with the switching element, and an electrode area. Each of the circuit patterns is of a shape generally similar to the ahspe of a figure xe2x80x9cLxe2x80x9d and extending along two sides of the insulator plate that are continued to and lie perpendicular to each other. The two circuit patterns are arranged at opposed corners of the insulator plate in a centrosymmetrical relation with respect to each other. The switching element is sandwiched between the free-wheel diode and the electrode area in each of the circuit patterns. The highly heat generating switching element is placed apart from corners of the back-side pattern. Accordingly, the semiconductor device of the present invention is highly resistive against the cracks in a binder between the back-side pattern and the metal base plate that occur at the corners of the back-side pattern by the effect of heat cycles caused by, for example, a start-and stop operation of the switching operation.
In accordance with a second aspect of the present invention, the semiconductor device has an auxiliary electrode that is connected to the electrode area. The width of the electrode area may be reduced by increasing the thickness of the auxiliary electrode. As a result, the size of each of the insulating substrate and the metal base plate is reduced. Therefore, a highly reliable and compact semiconductor device can be provided.
In accordance with a third aspect of the present invention, the two switching elements and the two free-wheel diodes are arranged in a checker pattern and are sandwiched by the two auxiliary electrodes placed along opposite sides of the insulator plate. Thus, the switching elements and the free-wheel diodes, which generate heat, may be spaced apart from all corners of the back-side pattern with no need to increase the size of the insulating substrate. Accordingly, the semiconductor device can be provided that is small-sized, compact and, however, highly resistive against the cracks in the binder by the heat cycles caused by, for example, the start-and-stop operation of the switching element.
In accordance with a fourth aspect of the present invention, the insulator plate is made of ceramics; the back-side pattern and the circuit patterns are made of copper or aluminum; the metal base plate is made of copper or aluminum; and the back-side pattern is bonded to the metal base plate by means of solder. This result in the semiconductor device having a superior cooling characteristic and capable of being produced at a low cost.
In accordance with the fifth aspect of the invention, the switching element is of a rectangular shape having sides of a length greater than 14 mm and is capable of being received in an area of 25 mm radius on a front face of the insulating substrate. Thus, the semiconductor device having a large capacity and a long life can be provided.
In accordance with the sixth aspect of the invention, the semiconductor device has a temperature sensor placed on said switching device at or near a corner of the back-side pattern. The temperature sensor can detect an extraordinary heating of the switching element before the element is broken by the heat. Therefore, the life of the semiconductor device can be accurately predicted. This increases the reliability of the semiconductor device.